Most current medical opinion assumes that neither silica exposure nor silicosis is related to lung cancer (Heppleston, 1985; Ziskind, et al., 1976) In the last six years, new experimental and epidemiologic research has emerged to challenge this position (Goldsmith, et al., 1982: Goldsmith, et al., 1986). The International Agency for Research on Cancer (IARC) has reviewed the carcinogenicity of silica and judged the evidence to be strongly suggestive. In particular, the epidemiologic research on the association between silicosis and excess lung cancer risk appears to be strong and consistent (IARC, 1987). However, almost all of this research has been conducted outside the United States. Furthermore, a critical evaluation indicates that some studies may be flawed due to inappropriate study design, confounding by smoking selection bias, competing causes, and insufficient latency times) Goldsmith and Guidotti, 1984). The present study will address these issues by conducting a follow-up mortality study of silicosis cases and matched and random controls from the California Silicosis Registry from 1950 to 1984: by determining if these cases have excess mortality from lung, lymphatic, stomach and other cancers; by assessing the validity of silicosis diagnosis through review of chest radiographs by a "B" reader and by assessing the relationship between length and type of employment and interaction with smoking among incident and deceased cases. This study will be the first research initiative on silicosis and lung cancer in the United States. There is a 90 percent likelihood that the estimated number of silicotic subjects (1,200 in the Registry is large enough to demonstrate a 4-fold excess risk. Successful completion of this study will benefit the 3,2000,000 American workers exposed to silica and approximately 100,000 U.S. silicotics who may currently be at greatly elevated risk of lung cancer and other cancers.